This invention relates to compositions for curing epoxide resins, to curable mixtures of these compositions and epoxide resins, and to cured products obtained by curing the aforesaid mixtures.
It is known that epoxide resins, i.e., substances containing on average more than one 1,2-epoxide group per molecule, may be cured by reaction with various classes of substances to form cross-linked, infusible, insoluble products having valuable technical properties. Typical curing agents include aliphatic, araliphatic, and heterocyclic-aliphatic amines, which are usually employed at room temperature.
For many applications, particularly as coatings and castings, it is required that the epoxide resins used have a low viscosity. This is usually achieved by the addition of diluents such as dibutyl phthalate and monoglycidyl ethers, e.g., iso-octyl glycidyl ether. Unfortunately, the addition of such diluents generally slows the rate of cure by the above-mentioned amines so that the mixtures take inconveniently long periods to gel. It has now been found that certain esters of phenols with carbonic acid may be mixed with epoxide resins both to lower the viscosity of the resins and also to accelerate the rate of cure with aliphatic amines at room temperature.
It has further been found that when such esters are used which have in the same molecule more than one carbonic acid ester group and also a long chain aliphatic group the resulting cured products have enhanced flexibility.
British patent specification No. 1,117,109 discloses that vic-alkylene carbonates may be incorporated into epoxide resin compositions where they act as solvents which lower the viscosity of the compositions and react with aliphatic amine curing agents to form carbamates. It is pointed out therein that there need be no weight loss during curing and no necessity for removal of volatile solvents. There is, however, no mention made that these alkylene carbonates have any accelerating action on the cure.
As is described in British patent specification No. 921,071, alkylene carbonates have been added to mixtures containing an epoxide resin, cellulose nitrate, and a curing agent for the epoxide resin in order to form subbing solutions. Such solutions are applied to hydrophobic, inert-surfaced film bases so that they will bond to a hydrophilic overcoating such as a non-curling layer or silver halide emulsion. The effect that addition of these alkylene carbonates has on viscosity and curing rate is not mentioned.
In West German patent application No. 2 459 752 there are described epoxide polyadducts prepared by heating an epoxide resin with cyanamide in the presence of a stabiliser which is a carboxylic acid or an alkyl or acyl ester of an organic or inorganic acid; diphenyl carbonate is among those esters which are preferred. As is generally known, cyanamide does not bring about the curing of epoxide resins at room temperature, heating to, e.g., 150.degree. C. being required to achieve cure in a reasonably short time. The presence of the stabiliser increases the pot-life of the epoxide resin-cyanamide mixture; this addition therefore has the opposite effect to that of accelerating the rate of cure.
According to Japanese Kokai No. 79.155298, epoxy resins are heat-cured with aryl esters, especially polyaryl esters, which term is used to include aryl carbonates. The curing action may be catalysed by addition of 0.1 to 10% by weight of a tertiary amine such as pyridine, tributylamine, and N,N-dimethylaniline. As is also generally known, tertiary amines are not effective room temperature-curing agents for epoxide resins. The effect of the addition of these esters is to extend the pot life of the mixture and so improve workability. In a comparative example, a mixture of an epoxide resin, triacetyl phloroglucinol, and tributylamine remained unreacted after 4 days at 20.degree. C. The effect sought is the opposite to that of accelerating the room temperature cure of epoxide resins.
The accelerating effect that has now been discovered could not be predicted by an examination of the above mentioned patents, since in none of them was such an effect suggested.